The present invention relates to control systems and methods for refrigeration systems, including air conditioners and heat pumps, which control systems avoid the need for expensive sensors and which are capable of functioning in a variety of refrigeration system models, without adjustment or selection. In this regard, the control methods and systems of the present invention may be termed "generic" in that a single control system is capable of serving a large number of different models, of widely differing capacities.
The present invention is particularly concerned with refrigeration systems of the type employed in air conditioners and heat pumps for cooling and heating living spaces. Such units are available in a wide variety of physical configurations and capacities, including small room air conditioners, self-contained reversible heat pump systems which somewhat resemble room air conditioners, but which provide both heating and cooling, central air conditioning systems which employ an indoor evaporator and a separate outdoor compressor/condenser combination, and similarly-configured heat pump systems which provide both heating and cooling by means of a reversible refrigeration system.
Such refrigeration systems, while apparently simple to control, in fact require fairly sophisticated control systems if proper operation and protection from damage under a wide variety of operating conditions, often adverse, are to be achieved. In addition, both heat pumps and air conditioners require periodic defrosting of the evaporator. For highest efficiency, defrosting should be done only when necessary.
Typical prior art control systems for protecting refrigeration systems employ a number of sensors so that the control system is provided with sufficient information upon which to base control decisions. For example, a common operating condition to which refrigeration systems are subjected is so-called "short cycling" which results when an attempt is made to restart the refrigerant compressor shortly after it has been running and before pressures within the closed circuit refrigeration system have had time to equalize. This condition typically results following a momentary power interruption, or as a result of user adjustment of a thermostatic control in a manner which causes the compressor to attempt a restart right after it has stopped. The compressor is unable to start under load, and hence stalls. Thus, typical control systems sense the overcurrent condition which results when the compressor motor is stalled, and de-energize the compressor motor for a cooling off period if the over current condition persists for more than a few seconds. Thermal overload protectors provide similar results.
A related adverse condition is simply a high load condition, which can result when power line voltage is excessively low (a so-called "brown out" condition), or when operating under extreme ambient temperature conditions. Thus, on an extremely hot day, an air conditioning system may be subjected to both a high load and low voltage. This tends to make the motor inefficient, which leads to over heating. Under such operating conditions, it is desirable to de-energize the compressor before damage results, and then allow operation to resume after a cooling-off interval.
Other compressor protection systems employ pressure sensors connected into the high-pressure side of the refrigeration system in order to sense excessive pressures, and de-energize the compressor when these occur.
By way of more specific example, various motor and compressor protection systems are disclosed in the following U.S. Patents: Anderson et al U.S. Pat. No. 4,038,061; Godfrey U.S. Pat. No. 4,079,432; Newell U.S. Pat. No. 4,253,130; and Genheimer et al U.S. Pat. No. 4,286,303. Of these, Anderson and Newell disclose relatively comprehensive systems for protecting air conditioners and heat pumps, and employ a variety of current and temperature sensors. Godfrey and Genheimer et al disclose motor protection systems in general which include the function of allowing a motor to attempt a restart following an overload, but only for a limited number of times.
Another approach to motor protection, particularly for a refrigeration system compressor motor, is disclosed in commonly-assigned Pohl U.S. Pat. No. 4,196,462. As disclosed in that patent, a single-phase AC induction motor of the type employing a capacitor-run winding can be protected from overload and overspeed conditions by monitoring the voltage across the capacitor-run winding. Under heavy loading conditions, the winding voltage decreases. This can be sensed, and used to initiate appropriate protection measures, such as a timed cooling-off interval. The system described in U.S. Pat. No. 4,196,462 also inherently recognizes a locked-rotor condition.
Defrost control systems typically employ from one to three temperature sensors in order to recognize particular conditions characteristic of excessive evaporator frost, and to initiate a defrosting operation when this occurs. Examples of such systems are disclosed in commonly-assigned Nolan et al U.S. Pat. No. 4,102,391 and commonly-assigned Pohl U.S. Pat. No. 4,215,554.
Another approach to detecting excessive ice formation on an evaporator, particularly the outdoor heat exchanger of a heat pump system, is disclosed in Gephart et al U.S. Pat. No. 4,123,792. Gephart et al recognize that ice buildup changes the loading on the evaporator fan motor. The degree of motor loading is monitored and detected by developing a signal proportional to the average product of motor current multiplied by the cosine of the phase angle between motor current and motor voltage.
In a related approach, Fowler U.S. Pat. No. 4,420,072 discloses a load indicator for a blower motor which circulates air through an air filter. When the filter becomes dirty, this condition is recognized by a change in motor loading.
From the foregoing brief background, it will be appreciated that prior art control systems not only require a relatively large number of diverse sensors, but also must be particularly adjusted to the size of the unit involved. Thus, overcurrent protection sized for a small air conditioner would be entirely inappropriate for a large one. By way of example, a typical product line may have from twenty to thirty different models, each requiring a customized control system.